Aims: The aim of this study was to examine whether the drinking habits of parents, siblings, and friends were related to regular drinking in adolescents and young adults, cross-sectionally as well as longitudinally. Methods: Data of 12-30-year-old twins from the Netherlands Twin Register were analysed. Information on regular drinking was collected in 1993, 1995, and 2000. Logistic regression analyses were conducted on cross-sectional data of 1993 (N = 3760), short-term longitudinal data of 1993-95 (N = 2919), and the long-term longitudinal data of 1993-2000 (N = 1779). Results: Results show that age, sex, and one's own previous drinking habits were important predictors of later-life regular drinking. Drinking habits of parents showed small but persistent positive associations. Alcohol use of the co-twin was strongly related to alcohol use of the participants, especially in the cross-sectional analyses, while alcohol use of additional siblings other than the co-twin was relatively unimportant. Cross-sectionally, friends' alcohol use showed a high association with regular drinking, but this association decreased over time. Conclusion: Cross-sectional analyses showed that a substantial part (29%) of the variance in regular drinking habits of adolescents and young adults was explained by the drinking habits of family members and friends, in particular, by drinking of co-twins and friends. But, over time, drinking by family members and friends could only explain a relatively small part (4-5%) of the variance in adolescents' and young adults' alcohol use. © The Author 2007. Published by Oxford University Press on behalf of the Medical Council on Alcohol.